The invention relates to toothed belts and sprockets, but more particularly, the invention relates to belts with curvilinear flank surfaces and sprockets for use therewith.
Toothed belts are extensively used in synchronous belt drives as substitutes for chain drives or gear trains. Unlike chains and gears that have pitch circles intersecting a point on a gear or sprocket tooth, belt drives have a pitch line that is displaced from both the belt and sprocket teeth and located at the belt tensile member. The displaced pitch line introduces the problem of assuring good entry of a belt tooth into a sprocket cavity with a minimum of interference at various belt loads and sprocket diameters. The belt-tooth/sprocket interference problem is compounded by: belt tooth-to-tooth pitch changes caused by elongation of the belt tensile member; deflection of elastomeric belt teeth; chordal spanning of the belt tensile member at or between sprocket teeth under load; and ratcheting at high-torque conditions on a belt drive where there is initially low static total tension on fixed center drives (i.e., without a means for belt tensioning). A trend in solving some of the belt-tooth/sprocket ratcheting and fit problems under various loadings, is to go from the traditional belt tooth having planar flank surfaces to a belt tooth having curvilinear flank surfaces.
There presently are several commercially available belts that have teeth with curvilinear flank surfaces. Such belts are represented and taught in U.S. Pat. Nos.: 4,452,594 to Patterson; 3,756,091 to Miller; 3,977,265 to Worley et al; and 3,924,481 to Gregg. While such belts offer viable solutions for transmitting power under an assortment of operating conditions, none provide the combined qualities of smooth-running and quietness with a resistance to tooth ratcheting under dynamic conditions, especially for drives with locked sprocket centers and high-tension ratios.
A yet uncommercialized and a theoretical attempt to combine features of smooth-running and antiratcheting is disclosed in U.S. Pat. No. 4,371,363 to Cicognani et al. The Cicognani disclosure does not present a plausible solution because the layout of the tooth profile is dependent on a coefficient for the particular type of elastomeric material forming the belt teeth. Consequently, such a design would not have universal commercial applications because of lack of interchangeability between belts made of different elastomeric material, as for example, belts made of rubber and higher modulus belts made of urethane.
Known belts having good antiratcheting capability under dynamic conditions are characterized by high teeth or teeth with steep flank surfaces, or a combination thereof. The antiratcheting belt exhibits some belt tooth to sprocket interference that usually generates noise with attendant belt vibration or rough-running. In contrast, smooth running belts with little running noise are characterized by shorter belt teeth that have shallower tooth flank surfaces that do not readily scrub against a sprocket.
FIGS. 1 through 4 are tracings of layouts that are computer generated for exactness of fit between belts and sprockets. FIGS. 1 and 2 are illustrative of belts with tooth profiles that inhibit ratcheting because of their larger tooth height to tooth base ratios and steep flank surfaces; however, both belts have attendant scrubbing between the belt teeth and sprocket. FIG. 3 is illustrative of a smooth operating belt with little potential for belt tooth to sprocket scrubbing because of its shallower flank surfaces and lower ratio of tooth height to tooth base; however, the belt exhibits a propensity to ratchet at less torque than the belts of FIGS. 1 and 2.
Referring to FIG. 1, the outline of a Patterson type belt PA having a tooth height H1 to tooth base B1 ratio of about 0.70 overlays a portion of a 32 tooth sprocket 12 to illustrate how the meshing characteristics of the belt depends on the shape of its flank surfaces. While there is no interference with a belt tooth 14 that is fully engaged in a sprocket cavity 16, the belt tooth 18 entering the sprocket groove 20 and belt tooth 22 leaving the sprocket groove 24 have flank surfaces 26, 28 that are so steep that they scrub against the sprocket teeth 30, 32 as is illustrated by the overlapped areas 34, 36. The interference causes noise and may induce unwanted belt vibration during belt operation. However, the belt PA exhibits excellent antiratcheting qualities that are hereafter tabulated in Table 1. If the sprocket teeth were narrowed for the purpose of facilitating tooth entry and exit without interference, as for example, cutting away the overlapped areas 34, 36, the belt would merely translate to a position against the sprocket teeth where there would still be belt tooth interference.
The belt HT as shown in FIG. 2, is a U.S. Pat. No. 3,756,091 type belt having a tooth height H2 to tooth width B2 ratio (i.e., tooth aspect ratio) of about 0.72. The operation of the belt HT is similar to that of the belt PA in that it exhibits similar antiratcheting qualities with some inherent belt tooth interference. While there is no interference with belt tooth 40 that is in full meshing engagement with a sprocket groove 42, a belt tooth 44 entering a sprocket groove 46 or a belt tooth 48 exiting a sprocket groove 50 has flank surfaces 52, 54 that scrub against sprocket teeth 56, 58 to the extent illustrated by the overlapped areas 60, 62. Scrubbing of the belt against the sprocket generates noise and potentially induces belt vibration. The steep flank surfaces in combination with the large aspect ratio (0.72) give the belt HT excellent antiratcheting qualities as comparatively summarized in Table 1.
The prior art belt of the U.S. Pat. No. 3,977,265 Worley et al type (not shown) has a tooth height to tooth base ratio of approximately 0.54; the belt has steep flank surfaces that are similar to the belt HT, which give it good antiratcheting qualities. However, the Worley et al belt has similar belt tooth to sprocket tooth interference problems as illustrated for the belt HT. Ratcheting characteristics of the Worley et al type belt are included in Table 1.
Referring to FIG. 3, a belt ST of the U.S. Pat. No. 3,924,481 type has excellent smooth-running qualities, but poor antiratcheting characteristics due to the combination of its relatively shallower belt tooth flank profile and tooth height H3 to tooth base B3 aspect ratio of approximately 0.58. Just like a belt tooth 66 fully engaged in a sprocket groove 68, a belt tooth 70 entering a sprocket groove 72 or a belt tooth 74 exiting a sprocket groove 76 exhibit no interference between flank surfaces 78, 80 and the sprocket teeth 82, 84. There are no overlapped areas in the vicinity 86, 88 to indicate scrubbing of the belt teeth against the sprocket teeth. Consequently, the belt ST exhibits excellent smoothrunning characteristics with little or no noise. However, the shallowness of the flank surfaces in combination with its lower tooth aspect ratio contribute to the belts poor antiratcheting qualities that are compared in Table 1.